The present invention is related in general to the field of semiconductor devices and processes and more specifically to the design, materials and fabrication of leadframes for high pin-count surface mount integrated circuit devices.
The leadframe for semiconductor devices was invented (U.S. Pat. Nos. 3,716,764 and 4,034,027) to serve several needs of semiconductor devices and their operation simultaneously: First of all, the leadframe provides a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip. Since the leadframe including the pads is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
Secondly, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the (xe2x80x9cinnerxe2x80x9d) tip of the segments and the conductor pads on the IC surface are typically bridged by thin metallic wires, individually bonded to the IC contact pads and the leadframe segments. Obviously, the technique of wire bonding implies that reliable welds can be formed at the (inner) segment tips.
Thirdly, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to xe2x80x9cother partsxe2x80x9d or the xe2x80x9coutside worldxe2x80x9d, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, this attachment is performed by soldering. Obviously, the technique of soldering implies that reliable wetting and solder contact can be performed at the (outer) segment tips.
Finally, the leadframe not only has to tolerate an encapsulation process at elevated temperatures, but also should be amenable to good adhesion to the encapsulating material wherever the leadframe and the encapsulating material share a common boundary. The adhesion should withstand thermomechanical stresses and prevent the ingress of unwanted moisture and chemicals.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, iron-nickel alloys for instance the so-called xe2x80x9cAlloy 42xe2x80x9d), and invar. The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
The trend in semiconductor technology is towards ever more input/output terminals for signal and power connections. This high pin-count trend has introduced the board attach method called xe2x80x9csurface mount technologyxe2x80x9d, which gradually supplanted the previously dominant trough-hole assembly. In the surface mount technology, the outer lead tips are either bent in xe2x80x9cJ formxe2x80x9d or, more popular, as xe2x80x9cgull wingsxe2x80x9d.
In support of high pin counts, the trend is further driving towards ever finer lead pitches. While the leadframe fabrication methods of both stamping and etching allow the manufacture of fine lead pitches, the success of surface mounting remains, in known technology, primarily limited by the capability of the soldering-process in board attachment. Lead pitches of 0.3 mm have been manufactured for the outer segment tips (with analogous inner segment tip pitches), and pitches of 0.15 mm have been proposed (for comparison, the diameter of a human hair falls in the range of 0.1 to 0.3 mm), but the solder attachment remains problematic.
While the wire bonding issues related to the fine-pitch inner segment tips has been aggressively addressed, only little work has been performed to solve the problems of fine-pitch outer segment tips. Molded packages having castellated periphery and lead arrangement allow a staggered positioningxe2x80x94and thus substantial physical separationxe2x80x94of the lead attachment pads on a wiring substrate or circuit board. The risk of electrical shorts by merging of the liquid solder is thus mitigated, but the cost of the precision-mechanical molds for fabricating fine-pitch packages is still prohibitive.
An urgent need has therefore arisen for a low-cost, reliable design approach for high lead count, fine-pitch IC leadframes which provides all the assembly features leadframes are expected to offer: Simple design, ease of chip assembly, bondability and solderability. The new leadframe and its method of fabrication should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability.
The semiconductor integrated circuit (IC) device according to the present invention comprises a planar leadframe having lead segments arranged in alternating order into first and second pluralities, the segments having their inner tips near the chip mount pad and their outer tips remote from the mount pad. The outer tips have a solderable surface. All outer tips are bent away from the leadframe plane into the direction towards the intended attachment locations on an outside substrate such that the first segment plurality forms an angle of about 70xc2x11xc2x0 from the plane and the second segment plurality forms an angle of about 75xc2x11xc2x0. Consequently, the outer tips create a staggered lead pattern suitable for solder attachment to an outside substrate.
The present invention is related to high density ICs, especially those having high numbers of inputs/outputs, and also to low end, low cost devices. These ICs can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. The package type can be small outline ICs (SOICs), quad flat packs (QFPs), thin QFPs (TQFPs), SSOPs, TSSOPS, TVSOPs, and other leadframe-based packages.
It is an aspect of the present invention to provide a leadframe with a dambar so that each dambar portion between the leadframe segments has a cut penetrating partially into the leadframe metal such that it is operable to open fully under the forces of the trim/form process.
Another aspect of the present invention is to provide the assembly of the IC device onto an outside wiring substrate or circuit board, wherein the substrate has rows of solderable attachment pads configured in a staggered pattern mirror-imaging the pattern of the leadframe segments of the device.
Another aspect of the invention is to provide a method of trimming and forming the leadframe strip made of a planar metal sheet. First, the length of the first plurality of the outer segments is trimmed, and then bent at an angle of about 70xc2x11xc2x0 against the leadframe plane. Second, the length of the second plurality of the outer segments is trimmed to a length different (usually shorter) compared to the length of the first segment plurality, and then bent at an angle of about 75xc2x11xc2x0 against the leadframe plane.
Another aspect of the invention is to bend the outer segment tips into a co-planar shape suitable for reliable attachment (usually by soldering) onto an outside wiring substrate.
Another aspect of the invention is to avoid the trimming of the dambar and rather replace this operation by the rupturing of the dambar portions, enabled by the pre-fabricated partial cuts in these portions. Proper bending of the two segment pluralities clearly separates the dambar portions in order to avoid electrical shorts.
These aspects have been achieved by the teachings of the invention concerning the geometries of the first and second pluralities of leadframe segments, the structure and rupturing method of the dambar, and the bending process and difference of the first and second leadframe pluralities.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.